When a magnetic-pole position of a synchronous machine is detected, a method of directly detecting an electric angle (magnetic-pole position) of a rotor using a position detector such as an encoder may be employed. However, in order to directly detect a rotation angle of the rotor, a sensor specialized for magnetic-pole position detection, such as a position detector, needs to be added to the synchronous machine. This causes disadvantages that the size of the apparatus configuration becomes large, and economic efficiency is also reduced. Therefore, apparatuses that detect a magnetic-pole position of a synchronous machine without using a position detector have been proposed.
As the methods for detecting a magnetic-pole position of a synchronous machine without using a position detector, for example, there are methods that use induced voltage or saliency of the synchronous machine.
In the case of position sensorless control using induced voltage, when the speed is zero, the position of a rotor cannot be precisely estimated since the induced voltage is also zero. In the case of a method using saliency, the saliency used for magnetic-pole position estimation varies with a period twice as long as that of the magnetic-pole position, and therefore the period of the estimated position is also twice as long as that of the magnetic-pole position. That is, the value of the estimated position when the magnetic-pole position of the synchronous machine is in a range of 0 to 180 degrees is the same as that when the magnetic-pole position of the synchronous machine is in a range of 180 to 360 degrees. Accordingly, this method is not sufficient to detect the magnetic-pole position for certain.
Therefore, at least when a synchronous machine is started up from the state where the speed is nearly zero, a method for estimating magnetic-pole position information of the synchronous machine is needed other than the method using saliency. As such a method, for example, Patent Document 1 discloses a method that uses magnetic saturation of a synchronous machine.
According to this method, 2n voltage vectors (n represents the number of phases and is a natural number larger than or equal to three) which have equal amplitudes and whose phases are shifted by equal amounts are applied to a synchronous machine, and added current values are obtained by adding up detection values of electric currents flowing when each pair of voltage vectors whose phase difference is 180 degrees is applied. A magnetic-pole position is detected based on the added current values. Thus, it is required that the synchronous machine enters magnetic saturation state due to the application of the voltage vectors.